Sunday, March 10, 2013

We've Got a Walker

Neha Patel

Imagine a world where a parasite
can enter your body and control your movements, only to be killed by the
invader exploding from your head. This sounds like something from a gruesome
sci-fi thriller, but for the Carpenter ant (Camponotus leonardi) this is a reality when it becomes infected with the parasitic fungus Ophiocordyceps unilateralis. These ants are popularly nicknamed "zombie ants" because of the alteration in behavior when infected. There are many different types of this genus of fungus, which affect different species of insects, but for this blog we will focus on the interaction between these two species.

The fungus O. unilateralisis found in tropical rainforests around the world, but much of the
studies have been done in Thailand. To observe the behavior of the infected
ants, Hughes et al. followed ants after they were taken over by the parasite. The parasite was ingested by the ants when they foraged in an area with the fungal spores. After
the C.
leonardi was infected, it went through
convulsions, causing it to fall down from the canopy away from the colony. This can be seen as
beneficial to the fungus, since ants will get rid of their dying brethren, and
the parasite might not survive to reproduce. The zombie ant then walked alone on low
vegetation, and further more only at a specific time during the day (9:30am-12:45pm).
They tended to walk on leaves, which is not a normal behavior for uninfected
ants. The ants would eventually bite down onto the underside of leaves, always around noon, and
would stay attached and die there. After dissection of the mandible, it
was shown that the fungus destroyed the muscle, which is what caused the ant
to lockjaw and hold fast onto the leaf. The fungus would then grow out and attach
more firmly to the leaf. Finally, a large hyphal stalk would grow from the back
of the ant’s head and release spores a few weeks later. The leaves the ants
attached to would always curiously be about 25cm above ground.

Planet Earth video showing a different species of ant (bullet ant) being body-snatched by a different Cordyceps fungus, complete with creepy music and all.

In 2009, researchers Andersen et
al. published an experiment that confirms the location of the final resting
place of the ant is very specific, and contributes to the optimal growth and
reproduction of the parasite. This change in behavior of the host is called an
extended phenotype of the parasite. To start their study, Andersen et al.
categorized where in the trees the ant graveyards were showing up, and what
part of the leaf the ants attached to. The infected ants were found about 25cm
from the ground, on the underside of a leaf, and 98% of the time biting a vein.
Then, to see if this place allows for optimal growth of the fungus from the
ant, infected dead ants were moved either all the way to the ground, or 15m up
the trees. Heights of the hyphal growths were measured in each place as a
measurement for reproductive success. Most of the ones displaced on the ground
disappeared, and the ones that remained showed stunted growth, inferring that
it wasn’t an ideal habitat for the fungus to grow. The ones higher up in the
canopy also had reduced height, possibly because the lower humidity and higher
temperatures caused desiccation. The authors concluded that it was probably no
coincidence that the average height of the final resting place was at 25cm,
which is a above the ground, and has high humidity and lower temperatures.

There is hope for these ants yet,
though! It has been noticed that these ants occupy canopies high up, which
allows them to avoid the ground where the fungus is waiting for them. Also,
they spend as limited time on the ground as they have to before climbing up
another tree. There seems to be a risk trade-off of braving the ground and the
current nutrient supply of the tree that the ants will consider. This is
suggested to be an adaptive response to this insidious and deadly fungus.

Much research has been done on
the behavior of the body-snatched ants, but not the mechanism of how the
fungus achieves this. Despite the fact that scientists have no idea how the
fungal parasite controls the behavior of its host, they are already trying to
research compounds produced by O. unilateralis for human use. The
compounds of interest are napthoquinones, which have been shown to have
antimalarial, and anti-tumor properties. In addition, in acidic environments,
they turn a bright red color. So, in 2005, Unagul et al. published a paper on
how to optimize the extraction of a napthoquinone compound called 3,5,8-TMON,
which has a potential of being used as a red dye in food and cosmetics.

Even though these compounds can
be useful for our lives, I don't know how I feel about getting them from a
fungus that turns something into a zombie. I'm probably consuming too much sci-fi and zombie media ("The Walking Dead"), but I'm scared culturing these in a
laboratory will lead to the body-snatching of humans. As a scientist, I know
there is a very low probability of this happening, but I am okay with being
ridiculous and paranoid!

Now, I leave you with a bad photo edit I put together of Rick Grimes of "The Walking Dead" being chased by zombie ants: